Probenecid does increase the retention of some substances. Those that have been researched are not banned substances. No substance is completely retained. Even those substances, mainly the penicillin-family of antibiotics, are better retained but never completely. Retention follows an exponential curve. The greater the dose of Probenecid, the greater the amount retained. There is no study that demonstrates complete or undetectable retention. Studies have only shown that Probenecid and the target substance are both detectable, even with old (1970) analytical machinery. The relationship between increase in dose of Probenecid and an interactive substance is exponential but never complete.

Probenecid is specifically designed with a molecular structure that has an affinity for substances with a similar parent molecule. Substances without the same parent molecule are not affected. If Probenecid is to increase retention, the substance needs to be of the like molecular structure. Most banned substances have different parent molecules and therefore would not be affected by its presence.

Probenecid has been shown to have some affinity with endogenous EPO. However, it is too big a step to assume it would have an affinity with exogenous EPO. The molecular structures of the two forms of EPO are different. That is why exogenous EPO can be detected and differentiated from endogenous EPO. It remains to be demonstrated that both forms of EPO are not differentiated by Probenecid. Until that is shown, the assumptive leap to say that Probenecid will cause measures of any EPO to be reduced is dubious at best.

No study has been performed with Probenecid and banned substances in a situation that would mimic its speculative use in sport settings.

The half-life of Probenecid is 4-6 hours, depending upon the dose. Consequently, if it was effective with a banned substance, it would have had to have been ingested just before testing took place. A few hours later, any partly retained substance would be "leaking" noticeably. But then, that assertion is based on the assumption of almost complete retention, something that has not been demonstrated by objective scientific research. The fact is that Probenecid does not work for very long. That is why in clinical settings for therapeutic purposes, it is given twice a day in moderate doses.

The reason Probenecid is on the banned list seems to be as follows. At the 1987 Pan American Games in Indianapolis, USA, a number of athletes recorded high levels of Probenecid. [Depending on who one talks to, the implied number of athletes ranges from quite small to a large group. Even this anecdotal evidence is inconsistent and unreliable]. It was assumed that those athletes were cheating. It is based on that suspicion at that time that Probenecid was added to the banned substances list. [Thisis the procedure for adding many substances to the banned list.]

Probenecid has been shown to work effectively, but not completely, with some antibiotics (penicillins and cephalosporins), with corticosterone, an anaesthetic (the name is not known to this writer), and endogenous EPO. There is one study that shows Probenecid stops the function of the banned diuretic (furosemide). That is close to the complete list of reported effects.

There is a need to use Probenecid in clinical settings. It allows smaller doses of antibiotics to be more effective longer, something that reduces the threat to the patient and increases the efficiency of a prescription. In Australia, it is considered a possible reason for "malpractice" if an individual is denied the use of Probenecid because he/she is an athlete.

It seems that many doctors associated with drug testing have stated so many times what Probenecid might do that they have come to believe the speculation is true (of course without any good science to back it up). It is common at hearings of athletes in Australia who have tested positive for Probenecid, and have documented proof that it was taken for clinical therapeutic purposes, that representatives of the ASDA state "Probenecid masks steroids". There is absolutely no proof that statement is true. This occurred at the ACB Anti-doping committee hearing for Graeme Rummans.